1. Field of the Invention
The present invention relates to a technique of generating a gamma curve, which is used for gamma correction of image signals in image display devices.
2. Description of the Related Art
Projectors, liquid crystal displays, and other image display devices use a gamma curve, which represents a relation between the input tone value and the output tone value of an image signal, for gamma correction. The gamma correction converts the tone value of an input image signal according to the gamma curve and thereby enables a displayed image to have substantially the same brightness property as that of an original image expressed by the input image signal. Some of the image display devices have an additional function of adjusting the gamma curve to display an image of the user's desired brightness property.
The gamma correction may be performed in image printing devices such as printers, as well as in the image display devices.
A known gamma curve adjustment method sets multiple control points corresponding to preset input tone values on a gamma curve and changes output tone values at the preset multiple control points to adjust a gamma curve as disclosed in Japanese Patent Laid-Open Gazette No. 2003-60914.
The conventional technique generates a gamma curve as a cubic spline curve passing through plural control points.
The conventional method of generating the gamma curve as the cubic spline curve requires specification of a cubic curve for each division between a pair of adjoining control points. For example, when there are 9 control points, the conventional method requires specification of total 8 cubic curves for 8 divisions.
The cubic curve is generally expressed by Equation (2) with four constants:f(x)=a+sx+tx2+ux3  (2)The conventional method thus requires determination of total 32 constants for specifying the 8 cubic curves.
When a CPU is used to specify the 8 cubic curves with determined 32 constants, the 32 constants are to be stored individually. Namely the conventional method requires 32 registers for storage of the 32 constants.
The conventional method of generating the gamma curve as the cubic spline curve requires determination of 4×(n−1) constants when there are ‘n’ control points. Coefficients of the spline curve are determined by solution of (n−1) simultaneous equations when there are ‘n’ control points. Namely the amount of computation for the spline cubic curve is proportional to the third power of ‘n’. A large number of control points demand an extremely large amount of computation and undesirably extend the total processing time. The required number of registers is also 4×(n−1) for storage of the determined 4×(n−1) constants. This undesirably expands the required circuit scale.